Cruise Control W126
 

There is a lot of talk about the cruise control unit under the dash, and how to fix it once it has gone south. There are photos on some of these threads as to what the circuitry looks like.

The fact is that many of the boxes are different. For reference's sake, I added a photo of the one found in our 1989 560 SEL. I have highlighed the capacitors that need replacement, and put their specifications into the photo. In my case they were six. Note that Radio Shack does not have these same ones. You can try ones with the same Farand specification, but higher voltages. Or you can go and get the precisely same rated capacitors at places such as US - Electronic Components Distributor | Newark.com (they are cheap).

Here is the picture (note that the 2 MicroF capacitor may actually be 2.2 MicroF - I am still trying to figure this out):

http://www.funnyyouasked.com/Images/...h%20labels.jpg

I will follow up as to effectiveness of the repair once the capacitors I ordered have arrived, have been installed, and all the other solder spots have been refreshed.

UDATE: By Micro I mean "μ", and the 2μ2 is indeed 2.2μ.

How far off were the old caps?

-J

Haven't pulled them yet & haven't tested. But I suspect it's the same thing that happens to everyone else. I re-soldered a few years ago and the system worked flawlessly for a couple of weeks - the caps must have gotten the "heat boost" that everyone is talking about, and then the thing started dropping out of Cruise Control randomly again.

Will post further - I do have a tester I can use on the caps...

I do as well, and my cruise is also dead. I should do a similar repair and add to this thread. I have my original 91 cruise module, as well as one from an 89 and as you noted they are not the same, IIRC the 91 module has fewer component parts on it.

-J

Although electrolytic capacitors are prone to drying out over time, the usual cause of intermittent cruise function is cracked solder connections at other capacitors. These are the red and green rectangular ones in the photo. Cracks are only visible under a magnifying glass once the conformal coating has been removed. Some people have resoldered the whole board but have not added fresh solder only to have the problem reappear at a later time or have not removed the conformal coating and end up with contaminated joints.

Right. I used a toothbrush and Acetone to remove the laquer.

However, my symptoms kind of match what I read on some other links: When you resolder only, chances are that the problem is also caused by old capacitors. The heat of the resoldering temporarily gives them a boost that then goes away. In my case the CC worked very well for a couple of weeks after I resoldered, and then started acting up again. Since it is a PIA to remove this thing, I am not putting it back in until I have put in new capacitors.

When I resoldered my module it worked for about 600 miles then quit again. I also took the actuator apart and cleaned the pot inside.

When I put it back together I didn't re-crimp the can fully onto the plastic base, so now when I want to futz with it I can just slide the board out the bottom. :cool:

-J

Cruise Control Repair
 

You guys have great patience and skill!

I just ordered a replacement off of Ebay. I hope it will work! It is the same part number as mine and off a newer 300E (1992).

Best of luck!

I am afraid it happens to all of them over time. You can also have it sent in to be fixed, there are people specializing in this. Capacitor replacement and so on. If the one you bought does not work, and you don't want to risk doing it yourself, send the unit in, and don't open it up beforehand!

This is what I did. After several re-solder attempts I decided to replace the caps. I had posted here, but did not have the cap specs I used. I noticed in a thread someone mentioned just re-soldering the caps. When I originally did that, it worked for a while. Twice. Then I replaced them. I think I now have a few thousand miles on the module. I put 22k miles on the car since I bought it last year and use the cruise often. So in my case, replacement worked well.

And for the record, I had used a solder removal tool to clear out older solder and use new. I usually always put fresh solder on things I freshen up.

Mouser Electronics - Electronic Component Distributor also has great pricing on electronic components.

BTW, if you become frustrated and throw in the towel, GDL rebuilds both the amplifier and the servo... General Development Laboratories - Table of Contents

Ya, but I'm not looking to spend $300+ on the cruise system.

I completely understand. I am just not wasting anymore time attempting to repair these cruise items. I just purchase reman units and be on with life. Hell, sometimes they are better than they were new.

On some of my cars I am forced into self repair due to lack of 'off the shelf' items available. Prime example are my grey market 745i BMW with their turbocharger aftercooler relay.

OK, I got some readings now, since I swapped them out:

22 μF 40v No 1: 23.58 μF (well within in the 20% threshold)
22 μF 40v No 2: 29.80 μF unstable (kept climbing, outside specs)
10 μF 63v No 1: 11.46 μF (well within in the 20% threshold)
10 μF 63v No 2: Several attempts no reading, then a couple of readings at 11.06 μF, but unstable (kept climbing, but at times would not read at all)
2.2 μF 63V No 1: 2.63 μF (just within specs)
2.2 μF 63v No 2: 2.597 μF (within specs)

With the unstable and intermittent 10 μF capacitor, and the crazy 22 μF capacitor readings, I think I may well have found the problem. Also, those marked 2μ2 are indeed 2.2 μF. Will let you know how the cruise performs once reinstalled. With the 10 μF capacitor working intermittently, it would make sense the cruise would work and then suddenly konk out.

(Note: I corrected this post to read from nF to μF!)

Quick update...
 

I tried testing the 10 μF again, and it would not read, after several attempts, while the others read just as I had described above. So now I am pretty sure the 10 μF was the culprit.